Fuel cells are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and oxygen containing oxidant reactant streams to power electrical apparatus such as motors, and transportation vehicles, etc. In fuel cells of the prior art, it has been discovered that, upon start up of fuel cells, corrosion takes place on catalyst layers of electrodes, and especially on cathode catalyst layers. That corrosion leads to performance loss of the cathode catalyst layers and the fuel cells.
In starting up known fuel cells that contain air on both anode and cathode catalyst layers and that employ a proton exchange membrane “PEM” as an electrolyte disposed between a cathode and anode catalyst layer, an oxygen containing oxidant is directed to flow through a cathode flow field that directs the oxidant to flow adjacent to the cathode catalyst layer. At about the same time a hydrogen rich reducing fluid fuel stream is directed to flow through an anode flow field that directs the fuel to flow adjacent the anode catalyst layer. As the fuel flows through the anode flow field, a fuel-air front is created moving along the anode catalyst layer until the fuel forces all of the air out of the anode flow field. It has been observed that the catalyst layer that is opposite the fuel-air front experiences substantial corrosion with each start up of a known fuel cell.
For example, FIG. 1 provides a schematic representation of a prior art fuel cell 10 showing chemical reactions that present a known understanding of how corrosion of a cathode catalyst layer 12 takes place. As is common, such a cathode catalyst layer 12 includes a catalyst secured to or integrated with a support material, such as platinum supported on surfaces of a porous carbon black. Upon start up of the fuel cell 10, a hydrogen rich fuel is introduced to an anode catalyst layer 14 in the left-hand side or “Region A” of the cell 10 as shown in FIG. 1, while the opposed cathode catalyst layer 12 is exposed to air. The hydrogen fuel dissociates into hydrogen ions and electrons, and the hydrogen ions pass through the electrolyte 16 from the anode catalyst layer to the cathode catalyst layer 12 within Region A. At the cathode 12, those hydrogen ions participate with electrons in the reduction of oxygen in the air to produce water.
In the right-hand side of FIG. 1, or Region B of the fuel cell 10 as shown in FIG. 1, the air on the anode catalyst layer 14 reacts with the electrons provided from Region A on the anode catalyst layer 14 and with the hydrogen ions or protons supplied from the opposed cathode catalyst layer 12 to form water. Provision of a flow of protons from Region B of the cathode catalyst layer 12 to Region B of the anode catalyst layer 14, and provision of a supply of electrons from Region B of the cathode catalyst layer 12 to Region A of the cathode catalyst layer 12 closes a circuit, raises the potential of the cathode catalyst layer 12 in Region B, and also results in a current reversal from a normal fuel cell operating mode. The reactions that occur in Region B of the cathode catalyst layer 12, as shown in FIG. 1, are corrosion of carbon to form carbon dioxide and electrolysis of water to form oxygen. The situation depicted in FIG. 1 also occurs when the hydrogen in the fuel stream is completely used. In such a circumstance, oxygen cross-over from the cathode catalyst layer 12 causes a localized “Region B” of FIG. 1 to form with resulting corrosion of the cathode catalyst layer 12 in Region B opposite the fuel starved location.
As is apparent, within Region B, a current reversal is effectively established by the reactions described in FIG. 1 that raises the local potential and rapidly degrades the carbon supporting the catalyst of the cathode catalyst layer 12 that is within Region B, or that is exposed to air and is opposed to an advancing fuel-air front on an anode catalyst layer of a known fuel cell. Examination of used fuel cells that experienced only a few dozen start up and shut down cycles showed that 25% to 30% of a high surface area carbon that supported the cathode catalyst of the cathode catalyst layer had been corroded away.
It is known that purging the anode and cathode flow fields with inert gases immediately upon shut down of the fuel cell passivates the anode and cathode catalyst layers to minimize such oxidative decay. However, use of inert purge gases gives rise to substantially increased complexity and cost of the fuel cell power plant that are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently. Another solution to the problem of start up corrosion is described in a U.S. Patent Application owned by the assignee of all rights in the present invention, which Application was published on Jun. 20, 2002 under number US-2002-0076582-A1. That solution proposes an extremely rapid purging of the anode flow field upon start up with the hydrogen rich reducing fluid fuel so that air is purged from the anode flow field in no more than one second, or as quickly as no more than 0.05 seconds. It is apparent that the mechanism leading to corrosion of the cathode catalyst layer and especially of the carbon supporting the catalyst layer positioned to be opposite the flow field having the fuel-air front occurs extremely rapidly during fuel cell start up. While known attempts to solve this problem have limited catalyst layer decay, it is still desirable to eliminate or further minimize cathode catalyst layer corrosion upon start up of a fuel cell.